Molybdenum disulfide is a known lubricant additive. Unfortunately, it has certain known disadvantages some of which are caused by its insolubility in lubricating oils. Therefore, oil-soluble molybdenum sulfide-containing compounds have been proposed and investigated as lubricant additives. For example, U.S. Pat. No. 2,951,040 discloses an oil-soluble molybdic xanthate as useful in lubricating compositions.
U.S. Pat. No. 3,419,589 discloses the use of certain "sulfurized" molybdenum (VI) dithiocarbamates as lubricant additives. These additives are described as being oil-soluble or at least capable of being easily suspended in oils.
U.S. Pat. No. 3,840,463 discloses the use of certain metal dithiocarbamates or dithiophosphates in combination with metal-free additives that contain sulfur and phosphorus.
U.S. Pat. No. 4,966,719, U.S. Pat. No. 4,995,996, and U.S. Pat. No. 4,978,464 all relate to the preparation and use of molybdenum-sulfur compounds. However, the materials described in those patents are prepared from starting materials that are themselves either relatively expensive or difficult to prepare.
The foregoing patents are listed as representative of the very many known lubricant additives containing both molybdenum and sulfur.
As is known in the art, some lubricant additives function as antiwear agents, some as antioxidants, some as friction modifiers, and some as extreme pressure agents. Indeed, some additives may satisfy more than one of these functions. For example, metal dithiophosphates represent a class of additives which are known to exhibit antioxidant and antiwear properties. The most commonly used additives in this class are the zinc dialkyldithiophosphates (ZDDP) which provide excellent oxidation resistance and exhibit superior antiwear properties. Unfortunately, they do not have the most desirable lubricity. Therefore, lubricating compositions containing these zinc compounds may also require inclusion of friction modifiers, which may lead to other problems, such as additive compatibility, in formulating effective lubricant compositions. Additives may lose their effectiveness when combined with incompatible additives in a lubricating composition. Generally, extreme care must be exercised in combining various additives to assure both compatibility and effectiveness. For example, some friction modifiers affect metal surfaces differently than antiwear agents do. When both are present, friction-reducing and antiwear additives may compete for the surface of the metal parts which are subject to lubrication. This competition can produce a lubricant that is less effective than is suggested by the individual properties of the additive components.
Trinuclear molybdenum compounds have been reported, but they are either ionic or have ligands with short chain alkyl groups. The reported compounds are consequently not oil soluble, and they have not been reported as lubricating oil additives.
Thus, there remains a need for improved lubricating oil additives that can be used with standard lubricating oils, are compatible with other conventional lubricant additives, and can be made from readily available starting materials.